As a result of their relatively light weight, excellent tear strength, tensile strength, flex life, abrasion resistance, and broad useful end-use temperature range, copolyester thermoplastic elastomers are used in a wide range of applications, including the replacement of metal components in many applications including motor and recreational vehicles, appliances, tools, electronics, furniture, and toys. It is desirable to have high temperature resistant structures made of thermoplastic elastomer materials for use in applications such as components for motorized vehicles and electrical/electronic devices. High temperatures may be routinely reached in the under-hood areas of vehicles, appliance wiring materials, offshore oil and gas applications, and inside laptop computers and cell phones. When these structures are exposed to high temperatures for prolonged periods, the mechanical properties of the article generally tend to be impacted due to the thermo-oxidation of the polymer component. This phenomenon is known as heat aging. International Patent Application Publication No. WO 2010/014796 discloses a method to improve heat aging characteristics of copolyetherester compositions through the use of additives comprising one or more polyhydric alcohols having more than two hydroxyl groups, in particular dipentaerythritol. The disclosed compositions are said to exhibit an improvement in the retention of mechanical properties when exposed to high temperatures.
It is also desirable to have flame retardant structures made of thermoplastic elastomer materials available for use in applications in the fields of construction, furniture, transport and electrical/electronic industries, so as to promote product safety, prevent the spread of fire and reduce destruction of products exposed to fire. The conventional practice for imparting flame retardancy to thermoplastic resin compositions has been the addition of one or more flame retardants or a flame retardant mixture. Such compositions typically include a halogenated organic compound such as brominated polystyrene as the flame retardant and an antimony compound as a synergist.
However, halogenated flame retardants tend to decompose or degrade at processing temperatures of thermoplastic elastomer materials, which implicates potential health and environmental effects due to release of gases. Consequently, the trend has been away from using halogenated compounds or mixtures containing them to impart flame retardancy.
Another conventional approach for imparting flame retardancy to thermoplastic resin compositions has been the addition of red phosphorus to the resins. International Patent Application Publication No. WO 92/20731 discloses a composition comprising an elastomer, a flame retardant comprising red phosphorus and ammonium phosphate as a flame retardant synergist. In addition, the use of fine red phosphorus powder homogeneously dispersed in the resin is known and practiced. A disadvantage of the use of this material is the well-known hazard of fire and explosion related to handling fine red phosphorus powders. Also, the combustion of red phosphorus causes emission of toxic fumes as a result of the formation of phosphine.
To avert the disadvantages and hazards of using halogenated flame retardants and red phosphorus, phosphinate salts, that is, salts of phosphinic acids, also known as phosphinates, have been substituted in thermoplastic resin compositions. German Patent Nos. 2,252,258 and 2,447,727 disclose phosphinates used as flame retardants. U.S. Pat. No. 4,180,495 discloses the use of poly(metal phosphinate salts) in flame retardant polyesters and polyamides. U.S. Pat. No. 6,255,371 discloses flame retardant compositions comprising a) phosphinates, diphosphinates, or polymers of these and b) condensation products of melamine, reaction products of melamine with phosphoric acid, reaction products of condensation products of melamine with phosphoric acid and/or mixtures of these. U.S. Pat. No. 6,270,560 discloses salt mixtures made from aluminum phosphinates, aluminum hydroxide, aluminum phosphonates and/or aluminum phosphates suitable as flame retardants for polymeric molding compositions. U.S. Pat. Nos. 5,780,534 and 6,013,707 disclose flame retardant polyester compositions containing calcium or aluminum salts of phosphinic acid or diphosphinic acid.
A disadvantage of using halogen-free, flame retardant compositions comprising copolyester thermoplastic elastomers based on the existing technology is that, upon exposure to high temperatures, the compositions may not exhibit the retention of mechanical properties necessary for highly demanding applications such as for example in automotive under-the-hood applications and in electrical/electronics applications. Such poor retention of mechanical properties may impair the durability and safety of the article containing the flame retardant copolyester thermoplastic elastomer with use and time.
Therefore, a need remains for halogen-free, flame retardant compositions comprising copolyester thermoplastic elastomers which exhibit the desired flame retardance and necessary mechanical resistance in the solid state, as reflected by retention of mechanical properties on high temperature long-term exposure.